Inkjet printers form printed images using one or more printheads that include arrays of inkjet ejectors. A controller in the printer operates the ejectors to form printed images that often include both text and graphics and may be formed using one or more ink colors. Some printer embodiments employ multiple printheads, each of which includes hundreds or thousands of ejectors. Multiple printheads form different portions of a printed image and, in multicolor printer configurations, different printheads emit different ink colors to form multicolor printed images.
During operation, some printers form printed test patterns that include predetermined arrangements of printed marks on an image receiving member, such as a print medium or an indirect image receiving member such as a rotating drum or endless belt. An optical sensor generates a two-dimensional set of scanned image data of the printed test pattern that a processor or other automated control device analyzes to identify the positions of inkjets and corresponding printheads in the print zone. In some instances, a printed test pattern does not include printed marks from an inoperable inkjet. As used herein, the term “inoperable inkjet” refers to an inkjet that either fails to eject any ink drops, only ejects ink drops intermittently, or ejects ink drops onto an incorrect location of the image receiving member. The processor identifies the positions of printheads to align and register the printheads to ensure that the inkjets in the printheads are in a predetermined position to form printed images with properly aligned ink drops. Additionally, the processor identifies inoperable inkjets for printhead maintenance operations that return the inkjets to operational order or that perform compensation processes for identified and deactivated inoperable inkjets during a print job.
While conventional printed test patterns and image analysis are effective for high-contrast printed test patterns, such as black or colored ink marks that are formed on a white paper substrate or metallic surface of an indirect image receiving member, some printing setups that use low-contrast inks and image receiving members cannot employ the prior art test pattern analysis techniques effectively. For example, in a printer that ejects white ink onto an optically transparent member, the scanned image data of conventional printed test patterns may not include sufficient contrast between the printed marks and the underlying background of the print medium to enable effective automated identification of the printed marks in the scanned image data. More generally, low-contrast printing configurations refer to combinations of ink colors, including optically transparent inks, and image receiving surface colors for which a printer produces scanned image data with comparatively small differences between the numeric values of image data pixels for the printed marks in the printed test pattern and the numeric pixel values of the scanned image receiving member. The prior art printed test pattern analysis techniques are generally less effective in the automated identification of the locations of printed marks in a low-contrast printing configuration. Consequently, improved systems and methods for the generation and analysis of printed test patterns that are formed using low-contrast ink and substrate combinations would be beneficial.